1. Technical Field
The present invention relates to a technique for producing a soft magnetic powdered core, preferably used for soft magnetic motor cores, rotors and yokes of motors in home appliances and industrial instruments, solenoid cores (stator cores) for solenoid valves installed in an electronically controlled fuel injector for a diesel engine or a gasoline engine, and the like, which require high magnetic flux density.
2. Background Art
Iron loss is a very important consideration in soft magnetic cores used in various actuators; it is defined by eddy current loss relating to a specific electric resistivity value of a core and hysteresis loss affected by strain in a soft magnetic powder, which is generated in a production process of the soft magnetic powder and subsequent processing steps. The iron loss W can be specifically defined by a sum of eddy current loss, We, and hysteresis loss, Wh, as shown in the following formula (1). In the formula (1), the first term represents eddy current loss We, and the second term represents hysteresis loss Wh. In this case, “f” represents the frequency, “Bm” represents the exciting magnetic flux density, “ρ” represents the specific electric resistivity value, “t” represents the thickness of a material, and “k1” and “k2” represent coefficients. Formula 1W=(k1Bm2t2/ρ)f2+k2Bm1.6f   (1)
As is clear from the formula (1), while the hysteresis loss Wh is proportional to the frequency f, the eddy current loss We is proportional to the square of the frequency f. Therefore, in a high frequency area, decrease of the eddy current loss We is effective in decreasing the iron loss W. In order to decrease the eddy current loss We, the specific electric resistivity value ρ should be increased so as to limit the eddy current in a small area. In this case, in a soft magnetic powdered core made from powder, for example, nonmagnetic resin can exist between soft magnetic powder particles such as iron powder. Therefore, the specific electric resistivity value ρ is increased, and the eddy current loss We can thereby be decreased. A conventional technique for producing a soft magnetic powdered core is disclosed in Japanese Patent Application of Laid-Open No. 2002-246219, in which a mixture of a soft magnetic powder and a resin powder is used, and compacting and heating are performed. In the soft magnetic powdered core disclosed in Japanese Patent Application of Laid-Open No. 2002-246219, resin exists between soft magnetic powder particles, whereby electrical insulation between the soft magnetic powder particles is specifically ensured. As a result, the eddy current loss We is decreased, and the soft magnetic powders are tightly bound, whereby strength of the soft magnetic powdered core is improved.
Conventionally, such a soft magnetic powdered core has been widely used because it is easily produced. When the above-described soft magnetic powdered core is used in a high frequency area, the electrical insulation is insufficient, whereby the specific electric resistivity value ρ is decreased, and the eddy current loss We is increased. The increase in the eddy current loss We causes heat generation, whereby resin binding the soft magnetic powders is deteriorated. Therefore, the soft magnetic powdered core has a disadvantage in that sufficient durability cannot be obtained. On the other hand, for example, when the resin amount is increased in order to improve the electrical insulation, the amount of the soft magnetic powder contained in the magnetic core (space factor) is decreased, whereby the magnetic flux density is decreased.
The soft magnetic powdered core may be used for electromagnetic actuators such as solenoids and motors. High attraction power and high responsiveness are required in an electromagnetic valve used in a fuel injector of a diesel engine. High magnetic flux density, high magnetic permeability, and small eddy current loss We in a high frequency area are preferable in stator core materials using the soft magnetic powdered core. Such a solenoid core is a soft magnetic powdered core obtained by compacting a mixture of iron powder and resin powder, and it is required to have high density and to have favorable electrical insulation between iron powder particles so as to increase the magnetic flux density and to decrease the iron loss.
On the other hand, reduction in size and high efficiency are required in various motors, and high magnetic flux density and small eddy current loss We in a high frequency area are preferable in rotor and stator materials using a soft magnetic powdered core. That is, required properties for a soft magnetic powdered core used in various electromagnetic actuators are substantially the same as those (specifically, iron loss) for a transformer core. In this case, a high magnetic flux density is required in the core of actuators because high attraction power is essential compared to the transformer core.
High density is necessary to obtain a soft magnetic powdered core with high magnetic flux density, and compacting pressure must be at least two times the pressure for producing ordinary sintered alloys. In a soft magnetic powdered core having a complicated shape or a thin shape, durability of a compacting die assembly would be deteriorated. Therefore, a soft magnetic powdered core having a shape similar to a solenoid core, a soft magnetic powdered core compacted to a simple cylindrical shape or a column shape is processed to have a predetermined shape and dimensions by machining. Alternatively, a soft magnetic powdered core compacted to a shape close to a product shape is machine finished at portions at which dimensional precision is specifically required. Therefore, the soft magnetic powdered core is required to have a good machinability, whereby wear of cutting tools can be minimal, and breakage and chipping of the material in machining can be prevented.
In view of the above circumstances, in order to decrease eddy current loss We and improve magnetic flux density B, various methods have been suggested (for example, see Japanese Patent Application of Laid-Open No. 2002-246219, Japanese Patent No. 3421944, Japanese Patent Application of Laid-Open No. 11-251131, and Japanese Patent Application of Laid-Open No. 2004-146804). In these methods, eddy current loss We is decreased by preliminary forming an insulating film on the surface of soft magnetic powders so as to ensure electrical insulation between the soft magnetic powder particles.
Since a magnetic flux density of a soft magnetic powdered core depends on the material density thereof, atomized iron powder is used as an iron powder so as to obtain high density, and the surface of the iron powder is coated with a film of phosphate compounds so as to decrease iron loss of the soft magnetic powdered core. As resin powders mixed with the iron powder, it is proposed to use a resin such as phenol, polyamide, epoxy, polyimide, and polyphenylenesulfide. For example, Japanese Patent Application of Laid-Open No. 2002-246219 discloses a soft magnetic powdered core obtained by adding 0.15 to 1 mass % of resin such as polyphenylenesulfide and thermosetting polyimide to atomized iron powders coated with phosphate compound. Japanese Patent No. 3421944 discloses a soft magnetic powdered core obtained by adding 2 mass % of thermosetting polyimide resin to atomized iron powder coated with phosphate compound. Moreover, in Japanese Patent Application of Laid-Open No. 11-251131, thickness of phosphate compound film is set to be not less than 10 nm and to be not more than 100 nm so that specific electric resistivity value ρ is not less than 2 Ocm and iron loss W is fixed. Furthermore, in Japanese Patent Application of Laid-Open No. 2004-146804, higher probability of resin powder existing between soft magnetic powder particles can be obtained by using resin powders having a small median size (median size of 30 μm or less), thereby obtaining soft magnetic powdered core having resin uniformly interposed between soft magnetic powder particles after heat treatment. The soft magnetic powdered core has eddy current loss We which is sufficiently small even when magnetic flux density B is improved by decreasing an additive amount of resin powder to 0.01 to 5 vol %.
As mentioned above, in a soft magnetic powdered core, eddy current loss We is decreased by improving electrical insulation, whereas magnetic flux density is improved by decreasing the additive amount of resin, and the soft magnetic powdered core has therefore been widely used recently. Furthermore, soft magnetic powdered cores, in which magnetic flux density is further improved while eddy current loss We is small, are needed.